This invention relates generally to the surveying of underground formations intersected by a well bore, utilizing injected radio-active tracers.
Radio-active tracer materials have been utilized to develop survey profiles of underground formations by injection thereof into a flow of steam introduced into a well bore through which the formations are surveyed. The measured intensity of radiation emitted from such injected tracer materials corresponds to the amount of steam entering the formation being logged at any given station within the well bore. Radiation measurement may be effected by means of a logging tool including, for example, a gamma ray detector such as those disclosed in my prior U.S. Pat. No. 4,096,752. However, measurement methods heretofore utilized involving a radio-active tracer such as I131 has been shown to be inaccurate.
Generally, logging tools have been inserted into the well bore through a tubing string, with its measurement output being delivered to above ground read-out equipment through a wire line extending upwardly through a stuffing box retaining well fluids within the tubing string. The stuffing box is connected to a lubricator pipe, within which the gamma ray detector is stored above ground. A riser fitting and a swab valve through which the logging tool is inserted into the tubing string, couple the lubricator pipe to the tubing string. Steam enters the tubing string below the swab valve while the tracer is injected through the riser fitting. Heretofore, a high pressure bleed valve was removably coupled to the riser fitting to vent well fluids and the lubricator pipe. The radio-active tracer material was also introduced with a pressurized fluid, such as nitrogen gas, through the bleed valve. With the foregoing arrangement, acceptable data was obtainable even though the riser fitting was a substantial distance from the location at which the steam is injected into the tubing string below the riser fitting.
Various drawbacks are associated with the foregoing apparatus arrangement for effecting the steam injection survey, including excessive loss of tracer material during venting and use of large quantities of nitrogen gas to carry the tracer material. Excessive pressure drop losses at elbow connections which distort the slugs of tracer materials injected and the splitting of tracer slugs during injection with over-pressurized nitrogen gas also made survey read-out data difficult to interpret. Also, problems have arisen involving the availability of fully operable bleeder valves. The use of bleeder valves that are not fully operable are a potential hazard.
It is therefore an important object of the present invention to provide apparatus and an associated survey procedure of the foregoing steam injection, tracer detection type which avoids the drawbacks and problems aforementioned.